A sheet feeder has a transmission mechanism which includes an output gear mesh with a transmission gear train, and a switch element, disposed between the transmission gear train and the output gear, for switching a rotational direction of the output gear. The switch element includes a movable arm, a driving gear driven by the transmission gear train, a switching gear set attached to the arm and meshing with the driving gear, and a clutch device for restricting movement of the arm to enable the switching gear set to be disengaged from the output gear. When the output gear meshes with the switching gear set, the output gear and a transmission shaft of the sheet feeding mechanism may change the direction of their rotation. When the output gear is disengaged from the switching gear set, the output gear and the transmission shaft of the sheet feeding mechanism are not driven by a power source.